The present invention relates to an improved method and apparatus for controllably applying a coating to particulate materials in a Wurster-type fluid bed agglomerating coater. The well-known Wurster apparatus and process is used for coating particulate material in a batch fluidized bed dryer. The fluid bed is widely used for drying powders, creating granules, or coating powders or granules of pharmaceutical, food and chemical ingredients. The Wurster process relies upon a continuous gas stream that moves the product (such as granules, beads, pellets, tablets) past one or more spray nozzles for purposes of coating, layering, and ultimately drying the finished product.
Typically, a Wurster apparatus comprises an outer container having one or more internal upwardly-extending cylindrical inner partitions or “Wurster tubes”, each suspended over a perforated gas distribution plate or screen which defines the bottom of the outer container. There is a gap between the bottom of the internal partitions and the distribution plate. The gap is adjustable. The internal cylindrical partitions divide the container into upbed regions inside the partitions surrounded by downbed regions outside of the internal partitions. The perforated plate is provided with larger and/or more numerous perforations within the area defined by the “Wurster tubes” for directing gas flow at a relatively higher volume and/or velocity into the upbed region, with the surrounding distribution plate area containing smaller and/or less numerous perforations for directing gas flow into the downbed region at a decreased velocity, thereby creating a recirculating flow within the apparatus which is upward inside of the partition, and downward on the outside of the partition.
Within the upbed region of the “Wurster tubes”, the higher velocity gas transports the particles of feed material past one centrally positioned spray nozzle for coating the passing particles of feed material with a coating solution. After passing through the spray, the particles enter a region of lower gas velocity in an expansion chamber portion of the apparatus above the partitions. When the particles have moved high enough in the expansion chamber, the gas velocity is no longer sufficient to support them, allowing them to fall into the downbed region where they return to the bottom of the chamber, pass through the gap at the bottom of the partition, and reenter the higher velocity gas to be repeatedly recirculated past the spray nozzle until they have been coated to the desired finished condition.
Various forms of the Wurster apparatus and process are disclosed in U.S. Pat. Nos. 2,648,609, 2,799,241, 3,089,824, 3,196,827, 3,207,824, and 3,253,944.
Previous patents have acknowledged that agglomeration can take place in a Wurster coater by particles entering into the wet spray region at the tip of the nozzle. Devices have been utilized to prevent particles from getting too close to the nozzle, to minimize the agglomeration that can take place. In most cases, the starting material to be coated consists of a range of particle sizes as some particle size distribution is inherent in the process used to create the material and, thus, contains a small percentage of small particles (fines). In addition, some small particles (fines) are generated in the materials handling steps preceding the Wurster coating process. In either case, small sized particles are generally not desirable. In addition, spray drying of the spray coating solution used in the process can introduce additional fines into the process stream.
Therefore, it is desirable to allow only the fines and smaller particles to agglomerate, and avoid the undesirable agglomeration of the larger particles. In this way the resulting coated product is of a more uniform size.